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Installing a Concrete Sidewalk Over Pavement

03/03/2009 10:30 AM

I'm preparing to design a concrete sidewalk on existing pavement, (Asphalt and Concrete). The sidewalk will be an island for pickup and drop off of pedestrians from a bus. My approach is to consider the sidewalk as a concrete overlay which is unbonded. This approach will allow the existing pavement and new sidewalk to work independently. My only concern is water and upheave. We plan to place 1/2" premolded joint filler where the curb meets the sidewalk and at intervals of 8 ft. It is common that this premolded joint filler deteriotes to a point where water will seep if not maintained. Since our approach is to build on top of existing pavement there will be no drainage layer underneath to allow water to fill the voids. I believe over time frost heave will damage the sidewalk.

Is this the right approach? Or should we remove the existing pavement and place a drainage layer followed by your typical 4" concrete. The existing pavement is currently designed for aircraft. The existing concrete slabs has reinforcement. We are also installing 6" steel faced curbs as per the architect.

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Join Date: Feb 2009
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#1

Re: Installing a Concrete Sidewalk Over Pavement

03/03/2009 4:00 PM

Personally I would remove the asphalt and put down your base/drain material and then pour your concrete. You are just asking for problems by pouring the slap on top of the asphalt. I would not be as concerned with the frost heave as I would be concerned with what the stopping of the heavy equipment (buses/cars), and the effect that would be with the "shoving" of the asphalt to the concrete. Picture a stop light with years of heavy traffic, and notice the ruts cause by the consistent stopping of the traffic. Now envision a concrete slab on top of the asphalt and the same problem occurring. My opinion would be remove the asphalt and pour.

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#2
In reply to #1

Re: Installing a Concrete Sidewalk Over Pavement

03/03/2009 4:45 PM

the only traffic that would be seen is pedestrian traffic. A 6" high steel faced curb will be installed and than the concrete behind it. The curb will be 20" deep. We propose to break the concrete or asphalt where the curb will be placed and place a 3 in. min. of DGA. The DGA will be placed behind the curb to the limit where the concrete will start. The concrete sidewalk will be 6 Inches. On the side where the curb meets the existing pavement...that will be backfilled with concrete up to existing grade.

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#3

Re: Installing a Concrete Sidewalk Over Pavement

03/03/2009 5:05 PM

When you say the existing pavement is asphalt and concrete, does that mean a two or three inch thickness of asphalt over a six, eight or ten inch thick reinforced concrete slab? How has the existing pavement performed to date? Is there sufficient slope so that water drains away freely. Are there construction joints in the concrete which could telegraph through into the new sidewalk? What soil conditions exist under the present pavement?

Can you include a rough sketch showing the extent of the new sidewalk and the extent of the existing pavement?

By the way, what is DGA?

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#5
In reply to #3

Re: Installing a Concrete Sidewalk Over Pavement

03/04/2009 12:41 PM

DGA = Dense graded aggregate

The pavement can support large aircraft grossing over 150,000 lbs. Typical these pavements for ashalt composition would entail 22" to 24" depth consisting of at least 5" bituminous concrete, DGA or Macadam, and structural sand material. Over the years I suspect that the area has seen several overlays to compensate for aircraft getting larger. The concrete pavement composition is also about 22". The PCC will be 15 to 16 inches with a DGA or Macadam base course on top of existing subbase.

We don't have borings for the exact location however, other locations on the airport showed a soil classification of SP, brown-medium fine sand. The water table is fairly high as its adjacent to a body of water.

The pavement area currently being used for storage and some pictures show that its in fair condition. The concrete slabs appear in fair condition as well. I'm trying to treat the sidewalk as a concrete overlay in which its unbonded. In this way the two materials act independantly and will minimize any reflective cracking. We are also placing a 1" bituminous concrete lift as a separation between the concrete pavements.

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#4

Re: Installing a Concrete Sidewalk Over Pavement

03/04/2009 11:29 AM

Was the pavement section you plan to overlyie with sidewalk constructed to carry vehicle traffic? If so the structural sections would be much heavier than any sidewalk design. You can just bore some drain holes or score through by saw cutting portions of the AC that underlie the proposed sidewalk just to get drainage. Put a thin layer of a couple of inches of drainage material like pea gravel, and place concrete on top. Alternately, you could pour right on the concrete, bore a few pins into the AC, and full thickness saw cut the joints through the AC. Since Concrete is way less permeable than AC, you should have minimal problems with water between the concrete and AC derived from permeation of the concrete. Howere, water will enter at the joints, so just make sure it can flow freely past the Concrete AC interface.

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#6
In reply to #4

Re: Installing a Concrete Sidewalk Over Pavement

03/04/2009 1:08 PM

Those are great alternatives. Let me see which would work best. We were thinking of drilling holes too, to allow the water to drain.

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#9
In reply to #6

Re: Installing a Concrete Sidewalk Over Pavement

03/05/2009 10:57 AM

Drilling holes through the pavement may not be a good idea. The material below is a fine sand which is also a frost susceptible material. Injecting water into that will tend to encourage frost growth below the pavement. If drainage can be handled above the pavement, it would be much better to avoid holes.

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#10
In reply to #9

Re: Installing a Concrete Sidewalk Over Pavement

03/05/2009 11:54 AM

Material below any properly designed pavement should not be a fine sand, but rather a permeable aggregate base rock. Remember, AC is permeable to water more so than concrete. So in effect all you are really doing is draining out water that collects near joints in the concrete. The subgrade of a pavement section is designed to handle draining water. Consider that they could just remove the AC, replace it with a agg base material (or scarify the AC and use it as a base) and pour directly on that, such that it would be way more permeable then the AC. The AC adds a performance characteristic under moving load equivalent to a much thicker sect of AB. He just needs the small amounts of water that drain through the concrete joints to drain away. (And even then i do not think he will collect enough water between the Concrete and AC to cause substantial frost heave.) If drainage collects behind the concrete up gradient on the AC surface they will need a means of removing that separately. However, something else that migth be a consideration, many airport have the AC surface underlain by thick sections of concrete (As AC is inadequate support for heavy aircraft, particularly on landings). You could have a very thick section of concrete underlying the AC.

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#7

Re: Installing a Concrete Sidewalk Over Pavement

03/04/2009 1:22 PM

My vote would be to build on top of the existing pavement, but I agree with RCE that water must be able to flow freely below the sidewalk. If the existing pavement does not have adequate gradient to remove the water from the area of the new sidewalk, alternative drainage arrangements must be made.

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#8

Re: Installing a Concrete Sidewalk Over Pavement

03/05/2009 9:53 AM

Could you use a layer of gravel for drainage, combined with heating elements to prevent freezing in icy conditions. The added expense might be worth it for a pedestrian area like that.

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#11

Re: Installing a Concrete Sidewalk Over Pavement

03/28/2009 4:02 AM

Brick Top the sidewalk and all the frost damage will be to grout and that is a quick and easy repair

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#12

Re: Installing a Concrete Sidewalk Over Pavement

05/16/2009 11:20 PM

You might suggest putting permeable concrete on as the top layer. This would allow the rain to peculate down through the sidewalk to the ground or runoff as the grade below the surface dictates.

Just a thought. Not great for every application, but still a tool to solve certain issues.

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